A membrane separation method, which involves allowing water to be treated to permeate through a separation membrane to remove components or particles from the water, is widely used in many fields, for example, in the purification of clean water, drinking water, industrial water, pure water, or the like, in the clarification of river water, sea water, or the like, in the treatment of various types of wastewater, human waste, sewage water, or the like, and in a water treatment, such as slurry condensation, of sludge or the like. Separation membranes for use in such applications are generally used in the form of a membrane module comprising a bundle of a plurality of membranes, fixed, e.g., in a case for collecting filtrate water.
Such a membrane module is required to have a large area for separation of water to be treated, to make good contact with the water, and to be capable of responding to a rapid change in the quality of the water. To address such requirements, which have a direct relationship to the quantity of water to be treated and the treatment efficiency, various types of membrane modules have been studied and put into practical use for microfiltration membranes and ultrafiltration membranes.
For example, a membrane module is known which comprises a cylindrical bundle of hollow fiber membranes, the both ends of the bundle being fixed, and a flow-straightening tube, having a large number of circular holes, disposed around one fixed end of the bundle of membranes. Water to be treated is allowed to flow into the membrane module from the circular holes (see, for example, Japanese Patent Laid-Open Publication No. H9-220446). The circular holes are also utilized as air inlets upon cleaning with air (air scrubbing) of the hollow fiber membranes.
In the hollow fiber membrane module having such a structure, however, there may occur a phenomenon (inter-fiber clogging phenomenon) of the formation of a rod-like bundle of hollow fiber membranes facing the circular holes, due to adhesion between the membranes in long-term use of the membrane module. When such a rod-like membrane bundle is formed, water to be treated hardly intrudes into the membrane bundle. Thus, this phenomenon leads to decreased filtration area and lowered filtration performance.
Further, air bubbles are likely to remain in the hollow fiber membrane module, which can cause membrane breakage. In particular, air bubbles are likely to remain in the upper end of the hollow membrane module upon air scrubbing, whereby an excessive stress is applied on the ends of the hollow membranes, causing breakage of the membranes at the ends. It is highly likely that such remaining air bubbles will cause a similar phenomenon upon reverse cleaning using filtrate water. Further, those portions of membranes on which air bubbles remain will dry and become hydrophobic. Accordingly, when filtration is resumed, water will not pass through those hydrophobic portions. Remaining air bubbles thus cause a decrease in the filtration area and a lowering of the amount of membrane-filtered water (flux).
In order to overcome such drawbacks, an immersion-type separation membrane module has been developed which comprises blind-like bundles of separation membranes, the bundles being arranged parallel to each other and fixed in a housing disposed in a water tank (see, for example, Japanese Patent Laid-Open Publication No. H5-220356). Such a separation membrane module, however, necessitates a larger installation area as compared to a cylindrical membrane module having the same filtration area. Saving of space, the inherent merit of a separation membrane module, can therefore be attained with difficulty. In addition, such a separation membrane module needs to use a large amount of cleaning chemical to clean contaminated membranes with a chemical, producing a large amount of chemical waste. This is unfavorable also in the light of increased environmental burden.